The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a socalled silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background.
The DTR-image can be formed in the image receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion (a socalled two-sheet DTR element) or in the image receiving layer of a socalled single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
Two types of the monosheet DTR imaging elements exist. According to a first type disclosed in e.g. U.S. Pat. No. 4,722,535 and GB-1,241,661 a support is provided in the order given with a silver halide emulsion layer and a layer containing physical development nuclei serving as the image-receiving layer. After informationwise exposure and development the imaging element is used as a printing plate without the removal of the emulsion layer.
According to a second type of monosheet DTR imaging element a hydrophilic support, mostly anodized aluminium, is provided in the order given with a layer containing physical development nuclei and a silver halide emulsion layer. Such type of lithographic printing plate is disclosed e.g. in U.S. Pat. No. 3,511,656. The lithographic plate is obtained by the following steps:
(a) imagewise exposing an imaging element comprising in the order given (i) a hydrophilic base, (ii) an image receiving layer containing physical development nuclei and (iii) a photosensitive layer containing a silver halide emulsion being in water permeable relationship with said image receiving layer,
(b) applying an aqueous alkaline solution to the imaging element in the presence of (a) developing agent(s) and (a) silver halide solvent(s) to form a silver image in said photosensitive layer and to allow unreduced silver halide or complexes formed thereof to diffuse imagewise from the photosensitive layer to said image receiving layer to produce therein a silver image,
(c) treating the imaging element with water to remove the layer(s) on top of said image receiving layer, thereby uncovering said silver image formed in said image receiving layer.
However, during the transfer between consecutive processing stations, the imaging element risks causing contamination of the solutions which would result in unwanted side effects. First, if any washing fluid or washsolution should pass to the diffusion transfer zone, the quality of the developed imaging element would not be equal over all places.
To avoid a significant part of these contamination problems, it is common practice to pass the imaging element between squeegee rollers. In order to further reduce such contamination problems, it has been described in Research Disclosure 15609, April 1977, to additionally apply flexible seals, made for example from commercially available extruded weather stripping material.